Conventional tester valves utilize annulus pressure to operate a valve element, such as a ball valve, where application of predetermined annulus pressure can be utilized to open the valve element while reduction of the annulus pressure can be utilized to close the valve element. One drawback to such a system is that the valve element will not remain in an open position when the annulus pressure is reduced. For certain downhole activities, however, it is desirable to hold a tester valve in such a “lock open” configuration once annulus pressure is reduced.
More recent tester valves employ mechanisms to lock open the valve element when annulus pressure is reduced. Specifically, a movable slotted sleeve is utilized to index the position of an actuation arm so that the actuation arm will not force the valve element to a closed position when the annulus pressure is relieved. While such systems may be functionally satisfactory, the systems utilized to apply the motivation force to move the slotted sleeve are complicated and often require operating pressures to activate the lock open feature that are significantly higher than the normal annulus pressure. For example, normal operating annulus pressures utilized with tester valves are typically in the range of 1200 psi, whereas annulus pressures of 2500 psi are required to operate lock-open features of certain prior art tester valves. Persons of ordinary skill in the art will appreciate that use of such high pressures with systems as described can adversely impact other components of the downhole mechanism, such as rupture disks, or system components with lower pressure ratings.
Accordingly, in view of the foregoing, there is a need in the art for a tester valve that utilizes lower annulus pressures to locked open a valve element. Such a tester valve would desirably utilize the same approximate annulus pressure to both operate the valve element and to lock open the valve element as desired.